1. Technical Field
The present invention relates to circuit devices, electronic apparatuses, and the like.
2. Related Art
Heretofore, a circuit device is known that realizes data transfer control of the USB (Universal-Serial-Bus). A known technology of such a circuit device includes a technology disclosed in U.S. Pat. No. 7,573,298.
A transmitting circuit that current-drives a first signal line and a second signal line that constitute a differential signal line is disclosed in U.S. Pat. No. 7,573,298. The transmitting circuit includes a constant current circuit that supplies a current to the first and second signal lines, a current control circuit that controls the current value of the current, a first terminating resistor circuit to be connected to the first signal line, and a second terminating resistor circuit to be connected to the second signal line. The current control circuit controls the current value of the current that the constant current circuit supplies to the first and second signal lines according to the resistance value of the first and second terminating resistor circuits.
With respect to the USB, an eye pattern is measured in the certification test. Therefore, the transmitting circuit for the HS mode of the USB needs to output a transmission signal that can pass the certification test of the USB standard with respect to the eye pattern. However, a parasitic capacitance and a parasitic resistance exist in the signal path of the transmission signal, and therefore a situation may arise in which passing the eye pattern certification test is difficult due to the parasitic capacitance and parasitic resistance.
A method is conceivable, as a method for improving the eye pattern during transmission, in which the eye pattern characteristics (such as a signal amplitude and a slope at rising or falling, for example) are adjusted by adjusting parameters (such as the resistance value of a terminating resistor and the current value of a constant current circuit, for example) of the transmitting circuit, for example. However, because a portion of the circuit (such as a terminating resistor, for example) is used for both transmission and reception, a situation arises in which, if the circuit parameters are optimized for transmission, an appropriate eye pattern cannot be realized in transmission and reception. For example, when the resistance value of the terminating resistor is reduced, the signal amplitude decreases in reception, and therefore a situation arises in which passing the USB standard certification test with respect to the eye pattern is not possible.
Note that the above-described problem is not limited to the USB, and it is possible that a similar problem may arise in serial communication in which a signal is transmitted through current-driving signal lines.